PCI/ATS: Consolidate ATS declarations in linux/pci-ats.h

Move ATS function prototypes from include/linux/pci.h to
include/linux/pci-ats.h as the ATS, PRI, and PASID interfaces are related
and are used only by the IOMMU drivers.  This effectively reverts
ff9bee89 ("PCI: Move ATS declarations to linux/pci.h so they're all
together").

Also, remove surplus forward declaration of struct pci_ats from
include/linux/pci.h, as it is no longer needed, since struct pci_ats was
embedded directly into struct pci_dev by d544d75a

 ("PCI: Embed ATS info
directly into struct pci_dev").

No functional changes intended.

Link: https://lore.kernel.org/r/20190914213032.22314-1-kw@linux.com
Signed-off-by: Krzysztof Wilczynski <kw@linux.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>